The present invention relates to integrated optical amplification devices, specifically, optical waveguides and lasers.
Optical communication systems based on glass optical fibers (GOF) allow communication signals to be transmitted not only over long distances with low attenuation, but also at extremely high data rates, or bandwidth capacity. This capability arises from the propagation of a single optical signal mode in the low-loss windows of glass located at the near-infrared wavelengths of 850, 1310, and 1550 nm. Since the introduction of erbium-doped fiber amplifiers (EDFAs), the last decade has witnessed the emergence of single-mode GOF as the standard data transmission medium for wide area networks (WANs), especially in terrestrial and transoceanic communication backbones. In addition, the bandwidth performance of single-mode GOF has been vastly enhanced by the development of dense wavelength division multiplexing (DWDM), which can couple up to 80 channels of different wavelengths of light into a single fiber, with each channel carrying up to 10 gigabits of data per second. Moreover, recently, a signal transmission of greater than 1 terabit (1012 bits) per second has been achieved over a single fiber on a 60-channel DWDM system. Bandwidth capacities are increasing at rates of as much as an order of magnitude per year.
The success of the single-mode GOF in long-haul communication backbones has given rise to the new technology of optical networking. The universal objective is to integrate voice video, and data streams over all-optical systems as communication signals make their way from WANs down to smaller local area networks (LANs) of Metro and Access networks, down to the curb (FTTC), home (FTTH), and finally arriving to the end user by fiber to the desktop (FTTD). Examples are the recent explosion of the Internet and use of the World Wide Web, which are demanding vastly higher bandwidth performance in short- and medium-distance applications. Yet, as the optical network nears the end user starting at the LAN stage, the network is characterized by numerous splittings of the input signal into many channels. This feature represents a fundamental problem for optical networks. Each time the input signal is split, the signal strength per channel is naturally reduced.
Rare earth doped optical amplifiers are emerging as the predominant optical signal amplification device for every aspect of optical communication networks spanning from repeaters, pre-amplifiers, and power boosters to in-line amplifiers for wavelength division multiplexed (WDM) systems. These amplifiers are suitable for long-haul, submarine, metro, community antenna television (CATV) and local area networks. An optical amplifier amplifies an optical signal directly in the optical domain without converting the signal into an electrical signal and reconverting the electrical signal back to an optical signal. As optical telecommunication networks push further and further toward the end user, as represented by the technology of FTTC, FTTH, and FTTD, there is an ever growing demand for compact and low cost optical amplification devices.
The key to an optical signal amplifier device is the gain medium. Gain media are typically made by doping rare earth ions into the core of an optical fiber. However, rare earth doped optical fiber has the disadvantage of high-cost, long length and difficulty of integration with other optical components, such as optical couplers, splitters, detectors, and diode lasers, resulting in high cost of manufacturing and bulkiness of the devices. As a cost-effective alternative to doped fibers, doped waveguides can be used as an amplification medium. Waveguides provide a benefit over fibers of being able to amplify a light signal over a significantly smaller area than fiber.
FIG. 1 shows a typical structure of a prior art integrated waveguide optical amplifier 20. The optical gain medium is formed by various processes (e.g. modified chemical vapor deposition, ion exchange, photolithography, flame-hydrolysis, reactive ion-etching, etc.) and the resulting gain medium is a straight line rare earth (RE) doped waveguide 22. The RE doped waveguide 22 is pumped by a pump laser 24, which generates a pump signal xcexp. Preferably, the pump laser 24 operates at approximately 980 nm, 1060 nm, or 1480 nm, although those skilled in the art will recognize that the pump laser 24 can operate at other wavelengths as well. The pump signal xcexp is combined with the optical signal xcexs to be amplified (e.g.1530 nm -1610 nm for an erbium doped channel waveguide) by a directional coupler 26. Optical isolators 28 are inserted into the optical path to prevent back-reflected signal amplification in the RE doped channel waveguide 22. The waveguide amplifier 20 may be used either as a signal amplifier as illustrated in FIG. 1 or as a laser 30 as illustrated in FIG. 2. In the latter case, reflection devices such as mirrors or fiber and waveguide gratings 32 are included in the optical path to create a laser oscillation cavity.
In order to achieve a desired 10 dB-30 dB signal gain in the amplifier 20, or to achieve laser output in the waveguide laser 30, a relatively high concentration of the rare earth ions are required, since the waveguide substrate (e.g. a four inch silicon wafer) can only accommodate a straight line waveguide with a length that is no longer than the waveguide substrate diameter. High concentration of rare earth ions can lead to problems such as ion clustering and lifetime quenching, which in turn reduce the amplifier performance. Furthermore, the straight line amplification waveguide can be required to be more than 10 cm long, which requires the dimension of the amplifier device to be greater than 10 cm in length, thus making it impractical to build the amplifier device more compact. Prior art as exemplified in U.S. Pat. No. 5,039,191 (Blonder et al.), U.S. Pat. No. 6,043,929 (Delavaux et al.), U.S. Pat. No. 5,119,460 (Bruce et al.), PCT Publication WO 00/05788 (Lawrence et al.), and J. Schmulovish, A. Wong, Y. H. Wong, P. C. Becker, A. J. Bruce, R. Adar xe2x80x9cEr3+ Glass Waveguide Amplifier at 1.55 xcexcm on Silicon,xe2x80x9d Electron. Lett., Vol. 28, pp. 1181-1182, 1992 all disclose such straight line waveguides.
It would be beneficial to have a curved channel waveguide that is contained on a relatively small area on a substrate, hence increasing the amplification channel waveguide length and reducing the overall size of the amplifier. Bruce et al. as well as M. Ohashi and K. Shiraki, xe2x80x9cBending Loss Effect on Signal Gain in an Er3+ Doped Fiber Amplifier,xe2x80x9d IEEE Photon. Technol. Lett., Vol. 4., pp. 192-194, 1992 disclose a curved zig-zag shaped channel waveguide 40 to increase the channel length, as shown in FIG. 3. However, this approach creates the problem of high bending losses at turning regions 42 in the curved waveguide 40. The bending radius is Rbending=(xc2xdn) Rsubstrate where n is the number of channel waveguide curve turning regions 42. Due to the high bending curvature, or small bending radius, the bending loss of such waveguide 40 is extremely high, resulting in low signal gain and limited usable waveguide channel length. Another approach is to use a spiral type waveguide with a plurality of 90xc2x0 bends to reduce the amount of area required for the waveguide, as is shown in FIG. 4. However, because of the tight bend radius at each of the 90xc2x0 bends, a substantial amount of light is lost at each bend.
Due to the disadvantages of the prior art described above, an optimized bending shape is desired to achieve more compact and integrated amplifier devices at lower manufacturing cost and without the losses exhibited by current curved waveguides.
Briefly, the present invention provides a channel optical waveguide. The channel optical waveguide comprises a substrate and an optical waveguide channel disposed on the substrate. The optical waveguide channel includes a first generally circular spiraling portion having a first free end and a first connected end, a second generally circular spiraling portion having a second free end and a second connected end, and a transition portion. The transition portion includes a first transition section connected to the first connected end, a second transition section connected to the second connected end, and an inflection between the first and second transition sections.
The present invention also provides a channel waveguide optical amplifier assembly. The assembly comprises a waveguide optical amplifier including a substrate, a first cladding layer disposed on the substrate and a second cladding layer disposed within the first cladding layer. The second cladding layer is generally annular and includes a generally tangential portion. The amplifier also includes an optical waveguide channel disposed within the second cladding layer. The optical waveguide channel includes a first generally circular spiraling portion having a first free end and a first connected end, a second generally circular spiraling portion having a second free end and a second connected end, and a transition portion having a first transition section connected to the first connected end, a second transition section connected to the second connected end, and an inflection between the first and second transition sections. The assembly also includes a pump laser disposed proximate the generally tangential portion and targeted to direct pump laser light to the second cladding layer through the generally tangential portion.
Additionally, the present invention also includes a method of amplifying light in an optical waveguide. The method comprises providing a channel optical waveguide, the waveguide including a substrate, a first cladding layer disposed on the substrate, a second cladding layer disposed within the first cladding layer, such that the second cladding layer includes a generally tangential portion, and a rare earth element containing optical waveguide channel disposed within the second cladding layer. The optical waveguide channel includes a first generally circular spiraling portion having a first free end and a first connected end, a second generally circular spiraling portion having a second free end and a second connected end, and a transition portion having a first transition section connected to the first connected end, a second transition section connected to the second connected end, and an inflection between the first and second transition sections. The method further comprises providing a pump laser disposed proximate the generally tangential portion and targeted to direct pump laser light to the second cladding layer through the generally tangential portion; transmitting signal light into the first free end of the first generally spiraling portion of the optical waveguide channel; and transmitting pump light into the second cladding layer through the generally tangential portion, the pump light exciting the rare earth element and amplifying the signal light.